The only things I think you really need are some stiffer front springs if running on grass, astro or carpet.
The rest of the parts are more tuning to the track or driving style than must have's in my opinion. I do now run the HRC rear block and 5 degree front hubs, but don't feel they are an essential upgrade straight off.
As for the diff, mine lasted longer with the kit items. I changed the balls to harder/ceramic ones and got new plates when it started to get crunchy, and with the harder balls the diff and thrust plates worn through in a meeting
